There is no requirement that suspense thriller movies be realistic. After all, one of the best of all time, North by Northwest, ended with its hero, an ordinary guy, dangling from Mount Rushmore. And nearly every movie Liam Neeson makes nowadays requires checking logic at the entrance to the cinema. But the difference between a good, albeit implausible, thriller and a bad one is whether the audience stops during the movie to think that the plot is simply too far fetched. And that’s the dilemma that the often suspenseful yet equally often frustrating thriller Bad Samaritan faces; it wanders over that plausibility line too often but almost makes up for it in goofy campiness.

The hero of Bad Samaritan is a most unlikely sort, Sean Falco (Robert Sheehan), a would-be photographer who works as a valet to make ends meet. More accurately, he uses his job as a valet parker as cover to rob some of the homes of the people whose cars he parks, finding where they live with their cars’ GPS systems and getting in with their keys. (In the first of many hard-to-swallow coincidences, many of these customers live within easy driving distance of the restaurant, so that Sean can break in, rob, and return before the owner finishes dinner.)

Unfortunately for Sean and his fellow valet partner in crime, Derek Sandoval (Carlito Olivero), they pick the wrong victim in Cale Erendreich (David Tennant). Cale is an obnoxious jerk (as shown in the clip below), but when Sean does get into Cale’s private office, he discovers that the jerk is also a serial killer and a kidnapper, who has a woman (Kerry Condon) bound and gagged in the room. Sean can’t get her free before he has to return the car, but he does call the police, first as an anonymous 9-1-1 caller, and then by showing up in person. However, when the police do investigate, they discover that Cale has removed all traces of the woman and any other possible crime. Instead, it’s Derek who falls under suspicion.

So far, so good, as far as thrillers go. In essence, up until this point Bad Samaritan is a takeoff on another Hitchcock classic, Rear Window, in which a man witnesses a crime but can’t get anyone to believe him, other than the killer himself. And that’s what happens here, as Cade figures out just who Sean is. But, instead of simply stonewalling the police with his credible denials while he goes about further torturing of his victim in, literally, his cabin in the woods hideout (where numerous previous victims have met their demises), Cade, and the movie, jump the shark. For Cade decides to seek revenge on Sean by harming Sean’s family and loved ones, including Derek (who astute viewers can probably guess won’t have a very long life expectancy in a movie like this) and Cade’s girlfriend.

It is at this moment that Bad Samaritan, which already required the audience to swallow an enormous amount of coincidence and unbelievable plot developments, completely goes off the rails. In the course of a few short scenes, Cade morphs from Hannibal Lector into Michael Myers, a crude killing machine wreaking rather gory havoc on several people. And, all the while this is going on, Cade keeps contacting the hapless Sean, gleefully telling him all the horrible things that will happen to him in the near future.

A glimpse at the behind-the-scenes talent involved in Bad Samaritan should give the audience somewhat of a clue as to how the movie goes off track. The film was directed by longtime Roland Emmerich protégé, Dean Devlin, who, after years of producing films with Emmerich, finally took up the directorial reins in Geostorm. Although David Tennant’s scenery chewing here resembles Gerard Butler’s efforts in the earlier film, Bad Samaritan actually seems more restrained in comparison.

In fact, Tennant’s performance here is so gloriously loopy that he keeps getting the audience involved in Bad Samaritan, no matter how implausible the storyline gets. Whether playing the smarmy jerk or ranting at Robert Sheehan or giving Kerry Condon lessons in how to conduct herself (here’s a hint to his psychopathy: he has a fixation on horses), Tennant is a heck of a lot of fun to watch. And, when Sheehan finally tracks him down at his rural cabin, the eventual showdown manages to be quite entertaining.

Devlin and screenwriter Brandon Boyce do manage to make Sean into an engaging, albeit flawed protagonist and not merely a foil for Cade’s machinations. The early parts of the movie, during which he tries to alert the police and, eventually, the FBI to the fact that Cade is a dangerous serial killer are among the better ones of that type that I have seen recently (although this movie, like numerous others of this genre, features police who seem exceptionally obtuse, with FBI official who aren’t much better).

Bad Samaritan walks the line between guilty pleasure thriller and just plain enjoyable thriller, in large part depending on how unhinged David Tennant’s performance is in any particular scene. I can’t whole heartedly recommend the movie because it comes off as ridiculously overwrought on too many occasions, but for those willing to forgive the frequent jarring credibility lapses and simply go along with it, rewarding scenes generally follow in very short order. Bad Samaritan isn’t that bad of a movie, and David Tennant proves very good at being very bad.